I double checked my owners manual again and all it says it that the heater will only work below 0F, and that above 32F you don't need to use a heater. They fail to explain what should happen BETWEEN 0 and 32.
There's no explanation in the book about setting off a DTC P0166.
The way that thing is described, sounds like they also put in a wimpy heater compared to previous years. Whoever designed this must not know what snow is.
Anyway, thanks for giving us this info. Makes me feel better now that I can replace the tumor plug with a normal one and not seriously damage anything.